Sparingly volatile fluids, such as, for example, ionic liquids or liquid polymers, are becoming more and more popular in process engineering because their low vapor pressure has various advantages. They are readily used as process solvents since the proportion of the organic impurities in the atmosphere (VOC) is reduced thereby; in addition, they are used as auxiliaries in the separation of substances.
For the industrial separation of mixtures of substances, multistage distillation under reflux, in short rectification, is frequently used. Key elements of rectification column are the evaporator in the bottom, by means of which an ascending vapor stream is produced, and the condenser at the top, by means of which a descending liquid stream is produced. Ascending vapor stream and descending liquid stream are brought into intensive contact with one another on internals. By means of the countercurrent thus produced, it is possible to adjust the composition of the streams at the top and bottom of the column within wide ranges with the aid of the column parameters height and reflux ratio when rectification section and stripping section are used. A large number of mixtures which cannot easily be separated by conventional rectification but preferably by extractive rectification [Stichlmair, S. and Fair, J., Distillation, ISBN 0-471-25241-7, Page 241 et seq., or Gmehling and Brehm, “Grundoperationen—Lehrbuch der Technischen Chemie, Band 2” [Basic operations—Textbook of Industrial Chemistry, volume 2], Thieme Verlag, 1996] occur in industry. This state of affairs is due to the similar boiling behavior of the components of the mixture, i.e. to their property of being distributed between the vapor phase and liquid phase in virtually identical or identical molar concentration ratio at a defined pressure and a defined temperature. A procedure frequently practiced in industry for separating close-boiling—the separation factor αij (ratio of the partition coefficients of the components i and j) of less than 1.2 is usually understood by this—or azeotropic systems is the addition of a selective additive, the so-called entrainer, in an extractive rectification. By selective interactions with one or more of the components of the mixture, a suitable additive influences the separation factor in the rectification so that the separation of the close-boiling or azeotropically boiling components of the mixture is permitted. The advantage of using a sparingly volatile liquid as an entrainer is as described in DE 10136614 and EP 1372807 for ionic liquids or in DE 10160518 for hyperbranched polymers—that the low vapor pressure of the entrainer prevents or minimizes contamination of the top component by the entrainer.
A second method which is frequently used in industry for separating azeotropic or close-boiling mixtures is liquid-liquid extraction [Sattler, K., Thermische Trennverfahren [Thermal separation methods], ISBN 3-527-28636-5, Chapter 6]. In this method, the liquid feed to be separated is passed countercurrently to a liquid, selective absorbing phase, the solvent in selected extraction columns. The intensive mass transfer between feed and absorbing phase results in the absorbing phase becoming enriched with one or more components of the feed and leaving the extraction column as an extract stream. The feed stream which is depleted in the components which have passed over into the extract stream is taken off as a raffinate stream from the extraction column. Both extract stream and raffinate stream can then be fed to separate rectification columns in which the respective stream can be separated into the individual components. Sparingly volatile liquids may expediently also be used as solvent as described in DE 10160518 for hyperbranched polymers or in “Ionic Liquids in Synthesis” (P. Wasserscheid and T. Welton, Wiley-VCH, ISBN 3-527-30515-7) for ionic liquids. Instead of extraction columns, other one-stage or multistage apparatuses, e.g. so-called mixer-settlers, are also used for liquid-liquid extraction.
Membrane separation methods constitute a third generic type of separation methods encountered in industry. The membrane separation methods utilize the fact that some components are transported through a membrane from a fluid feed stream more rapidly than other components. In this way, a permeate stream is obtained behind the membrane and, analogously to the liquid-liquid extraction, a retentate stream depleted in at least one component. A solvent can be used behind the membrane in order to improve the separation effect (so-called pertraction—permeation and extraction) or in order to be able to use an electric field, such as, for example, membrane electrophoresis or electrofiltration/electrodialysis, where an electric field is applied across the membrane in order selectively to influence the transport of substances through the membrane in a desired manner (Membranverfahren [Membrane methods], T. Melin and R. Rautenbach, Springer-Verlag, 2004). Sparingly volatile liquids may also expediently be used as solvent.
For cost reasons, it is always desirable to minimize the amount of entrainer to be used in extractive rectification or the amount of solvent to be used in liquid-liquid extraction or a membrane method and to recycle it to the separation process after purification.
Problems occur if, for example in a purification rectification associated with extractor rectification or liquid-liquid extraction or the membrane method, a sparingly volatile entrainer or sparingly volatile solvent has to be freed from all more readily volatile impurities. Usually, complete purification of the entrainer or solvent is necessary since residues of readily volatile impurities hinder the main separation process. The sparingly volatile liquids cannot be purified to any desired high purities since there would then no longer be any fluids at all in the bottom of the column which would have a vapor pressure which could be utilized for evaporation. Since in this case only the sparingly volatile component would be present in the bottom of the column, it would be necessary to set either temperatures which are so high as to be technically unrealizable and/or pressures which are so low so as to be technically unrealizable. The abovementioned patents accordingly explain “regeneration of the entrainers ( . . . ) by a stripping column. Since the vapor pressure of the (pure) entrainer ( . . . ) and hence also its partial pressure in the mixture with the bottom product are zero, the entrainer ( . . . ) cannot be freed completely of the bottom product by pure evaporation in the counter-current process.” (DE 10136614 A1, page 3, line 60 et seq., DE 10160518 A1, page 4, line 19 et seq., EP 1372807 B1, page 10, line 46 et seq.).
The same problem arises in a (one-stage) distillation, evaporation or flashing for freeing the sparingly volatile component of all more readily volatile impurities. In Chemical & Engineering News (American Chemical Society) of Apr. 29, 2002, page 4, Prof. Albrecht Salzer writes “that IL (ionic liquids) cannot be distilled, which would be a simple way of recycling, but have to be extracted with organic solvents”.
Analogous problems arise in chemical reactions which are carried out in sparingly volatile fluids (e.g. ionic liquids) e.g. described in: “Ionic Liquids in Synthesis” (P. Wasserscheid and T. Welton, Wiley-VCH, ISBN 3-527-30515-7). Here, various volatile substances, in particular reaction products, have to be separated from the sparingly volatile fluids. The more successful the separation of the products, the higher is the direct yield and the greater is the conversion on re-using the recycled solvent in equilibrium reactions.
In the abovementioned and further publications (e.g. WO A 99/41752, US 2003 0085156 A), in which sparingly volatile fluids are used as selective additives for the separation of substances, known alternative methods for freeing the sparingly volatile fluids from impurities are proposed, but without discussing them explicitly, e.g. according to thermophysical properties of the substances involved:                stripping with vapors or gasses        extraction with liquids or supercritical gasses        fractional crystallization/precipitation of individual components        electrolysis/electrochemistry        preparative chromatography        chemical reactions before separation        
WO A 2001 15175 describes a method for purifying ionic liquids in which the liquids to be purified are thermally decomposed at low pressure and the decomposition products are purified and are reacted back to give the ionic liquid again. However, losses of product occur. In addition, the preparation of ionic liquids and hence also the reconversion of them from the decomposition products are extremely expensive.
It is known to the person skilled in the art that both the use of auxiliaries to be removed again or optionally to be purified (stripping, extraction), the handling of solids (crystallization, electrolysis, chromatography) and the use of chemical reactions are associated with considerable effort, which is to be avoided.
Accordingly, the use of sparingly volatile fluid is currently limited by virtue of the fact that no industrially suitable purification methods are available.